Fix missing documentation for several GDI functions.

The documentation for wxGetClientDisplayRect(), wxGetDisplayPPI(),
wxGetDisplaySize() and wxGetDisplaySizeMM() didn't appear in the output
because they were documented as "overloads" of the corresponding functions
without "Get" prefix inside an @addtogroup Doxygen macro and apparently this
is not supported.

Simply document them separately to fix this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74316 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775